Indicates to use the cross compiler we have just built
instead of the one in /usr/bin.

--disable-libstdcxx-threads

Since we have not yet built the C threads library, the C++
one cannot be built either.

--disable-libstdcxx-pch

This switch prevents the installation of precompiled include
files, which are not needed at this stage.

--with-gxx-include-dir=/tools/$LFS_TGT/include/c++/7.2.0

This is the location where the standard include files are
searched by the C++ compiler. In a normal build, this
information is automatically passed to the Libstdc++
configure
options from the top level directory. In our case, this
information must be explicitly given.